The present invention relates to the fabrication and handling of precast concrete panels such are used in "tilt-up" construction and in forming other types of structures, such as retaining walls. Concrete panels of this type are generally flat and are formed by pouring concrete into a relatively shallow, horizontally-oriented mold, allowing the concrete to set, and then removing the panel from the mold. Such panel casting may be carried out at a central plant, but it is most often done using molds made from wooden forms at the job site.
The invention is specifically directed to the fabrication of such panels, which may weigh many tons, wherein lifting anchors are cast into one end or side edge of the panel. A lifting anchor includes an elongated shank having an enlarged foot by which it is anchored inside the panel, and an enlarged head which is positioned within a recess formed in the edge of the panel. Lifting hardware designed to grasp the recessed head of the anchor includes bails which accept crane hooks, so that a concrete panel can be lifted by its anchors utilizing a standard crane hook. An example of such ground release hardware is disclosed in our commonly assigned U.S. Pat. No. 4,700,979.
When an anchor is cast within a tilt-up panel, it is desirable to position the anchor with its head spaced inside the edge face of the panel, in a recess formed during the casting process. After the precast panel has been moved to its final position, by means of lifting hardware and the anchors, the recess can be easily patched to avoid rusting of the anchor and resulting surface staining of the concrete surrounding the anchor head.
Recess plugs are commonly used both to form hemispherical recesses in the panel and also to hold the anchor in the proper recessed position relative to the panel formwork during the concrete casting process. An example of such a recess plug is disclosed in Haeussler U.S. Pat. No. 4,296,909 of 1981 and includes a pair of quarter-spherical plug parts which are attached to each other by a hinge assembly that also supports a threaded rod. Each of these half-plug parts has a flat front wall for abutting a flat formwork face, and the threaded rod extends perpendicularly through a hole in the formwork so that the plug can be attached to the formwork by threading a nut onto the bolt on the outside of the formwork. Each of the plug halves includes a notch leading into an interior recess, and when they are joined together, these notches and recesses define a cavity which is shaped to receive the flared head and adjacent portion of the shank of a lifting anchor.
When the plug is attached to the wall of a formwork section, the engagement with the wall clamps the halves together and holds them around a lifting anchor so that the anchor extends perpendicularly to the formwork wall. After the concrete is poured into the form and hardens, the formwork is stripped away, and the plug is removed to leave the anchor head positioned within a hemispherical recess.
A disadvantage with recess plugs of this type is that they lack means for positioning reinforcing bar means in the panel which will be in the proper relation to the head of each lifting anchor, since it is this region of the panel which must bear the shear forces that the lifting structure imposes on the concrete as the panel is being tilted up from a horizontal position. More specifically, the lifting force applied to the anchor at the beginning of a tilt-up operation is translated into a severe shear force imposed on the concrete lying between the head and adjacent shank portion of the anchor. Unless this force is transferred to the mass of concrete lying below the anchor, spalling and cracking of the edge portion of the panel is likely to occur.
There is therefore a need in the industry for recess plugs which are relatively inexpensive to manufacture, which will function to locate reinforcing bars in properly close relation to each lifting anchor, and which can be attached directly to formwork without having to drill relatively large bore holes through the formwork, as with the plugs of the above Haeussler patent. Not only does this require additional labor, but formwork used for different jobs may require different locations for the lifting anchors, and previously bored holes which are not used for the next job must be patched in order to prevent leaking of concrete through the holes and corresponding defacing of the surface of the panel.